Solar radiation (sunlight) is the main provider of energy for the melting
of snow and ice in Greenland. As the solar radiation reaches the ice sheet,
a large percentage is reflected back up; fresh snow reflects up to 90% of
sunlight. The ice reflectivity (a.k.a. albedo) therefore exhibits a strong
control on the melting of the ice sheet. However, Greenland ice albedo has
been decreasing since the beginning of satellite observations in 1981.

The darkening has several causes. Firstly, snow crystals undergo
metamorphism which makes them poorer reflectors, a process that accelerates
with increasing temperature. So a warmer summer will cause a darker ice
sheet surface. Also, since some of the previous summers were
record-breaking in terms of temperature and melt, areas that were snow
covered now reveal darker bare ice, and areas that were ice before, now
are exposed earlier in the melt season as the winter snow blanket
disappears earlier. In addition to these factors, the ice sheet albedo
is also impacted by dust, transported from close by and far away, particles
released into the atmosphere by increasing amounts of wild fires in North
America, pollen, and industrial emissions, such as those by ships taking a
northerly route.

The darkening of the ice sheet is a self-reinforcing mechanism called the
melt-albedo feedback. The darker the ice sheet gets, the more solar
radiation is absorbed, which in turn facilitates melting and snow
metamorphosis. This results in more exposed ‘dark’ ice, or a reduction in
snow albedo.